1. Field of the Invention
This invention relates to a linear actuator and, more particularly to a linear actuator having travel limit function.
2. Description of the Prior Art
A typical actuator of this kind comprises a motor, a screw spindle and a screw nut, a driving tube engaged to the screw nut, and an external tube, which protects the driving tube. In many applications, when the driving tube reaches one of the two preset extreme positions, it is often required that the driving tube is stopped moving.
One known actuator of this type is as illustrated in FIG. 5. A driving tube 3, engaged with a screw nut 4, surrounded by an external tube 2, and two limit switches: shortening limit switch 61 and extending limit switch 62, mounted on a circuit board 6, are located at two extreme positions of the actuator. When driving tube 3 reaches one of the extreme positions, screw nut 4 will trigger one of the limit switches and cut off the motor current, and therefore the driving tube will stop moving outwards or inwards. One of the drawbacks of this solution is that the installation of these limit switches and its control circuit requires a large external tube, and hence increases the dimension of this actuator. Furthermore, assembling limit switches into the external tube is quite time consuming. Finally, for each travel length, a different circuit board is required. This increases part variety, and hence production cost.
The second solution, which implements travel limit function, is by counting the rotational turns of screw spindle. A gear train being connected to the screw spindle, for instance, is used for this counting purpose. When the number of turns is larger than a preset value, i. e. the actuator reaches one of the extreme positions, the gear train will turn off the motor electrically or mechanically. Similar to the previous approach, the gear train needs to be altered when actuator stroke is changed.
The third solution, which implements travel limit function, is by monitoring the current consumed by motor of the actuator. When the driving tube reaches one of the extreme positions, the driving tube or the screw nut will begin to move against an obstacle, and motor current increases tremendously. When the monitored current is larger than the preset value, a control circuit will turn off the motor. A pitfall of this over-current approach is that before motor current reaches the preset current limit, motor torque might already damage the power transmission components, such as screw spindle and nut. This solution requires precise tuning of the current limit for each actuator, and thus increases manufacturing cost.
A further solution, as disclosed in EP 0647799A2, uses internal projections, a spring clip 23 and a bushing 22 as shown in FIG. 6, surrounded by an external tube 2 are used to trigger two opposite oriented limit switches: shortening limit switch 61 and extending limit switch 62. When driving tube 3 reaches shortening limit position, screw nut 4 will push against spring clip 23 and thereby pull external tube 2, which will press the trigger button of the shortening limit switch 61. This will disconnect the motor current and stop driving tube 3. When driving tube 3 reaches extending limit position, screw nut 4 will push the bushing 22, and thereby pull external tube 2, which will release the trigger button of the extending limit switch 62 and stop motor 9. When driving tube 3 is subjected to a vertical force, external tube 2, due to assembly clearance, will bent to one side and might miss the two opposite oriented limit switches 61 and 62 as shown in FIG. 7. This might cause the failure of the travel limit function. Choosing a tighter tolerance will increase friction between external tube 2 and driving tube 3 and indirectly increases power consumption of motor, and also the assembly cost. Furthermore, triggering limit switches solely through projections inside the external tube results in high manufacturing and assembly cost of the external tube.